alry division, although not neglected, remained more or less a theoretical unit. As the Army mobilized for the Mexican border crisis and took note of trends in foreign armies during the initial campaigns of World War I, its leaders became increasingly convinced of the need to create permanent tactical divisions. Congress approved them in 1916, but the nation entered World War I before these plans had been perfected.
Events during the next two years, however, profoundly affected divisional organizations, the infantry division in particular. For the first time in the nation's experience, the United States Army mobilized a huge expeditionary force to fight overseas in Western Europe, a mission for which it was thoroughly unprepared. The day of the old constabulary army was over. Faced with threats to national security of hitherto unimagined scope emanating from the Old World, the nation had to revolutionize its army to wage war against a formidable continental opponent. The necessity for an effective combined arms organization would force extraordinary changes in its entire structure.
